1. Field of the Invention
The present invention discloses a plastic keypad and a process for the manufacture of the plastic keypad. The keypad is made of a) a formed plastic shell, b) graphics printed on the interior surface of the formed plastic shell; and c) a plastic filling the hollow cavity or cavities of the formed plastic shell. The formed plastic shell of the keypad is a polycarbonate and the plastic filling the hollow cavities is a thermoplastic resin or an elastomer. Additionally, the background printed on the interior surface of the formed plastic shell is made with a flexible ink and the keypad has a wear resistance rating in a Norman tester of greater than 40 cycles before the ink on the keypad fails on abrasion testing.
The process for the manufacture of the plastic keypad comprises a) second surface printing of graphics and background onto a flat plastic sheet, where ultraviolet-light curable inks are used for the printing of the graphics and flexible inks are used for the printing of the background; b) forming the flat plastic sheet into a formed keypad, where the formed keypad has at least one button having a hollow cavity on an interior surface; c) filling the interior surface of the hollow cavity with a plastic; and d) trimming or profiling the formed keypad to the specifications of the manufacturer. The flat plastic sheet is a polycarbonate and the plastic used to fill the interior surface of the hollow cavity is a thermoplastic resin or an elastomer.
2. Description of the Related Art
Keypads for data input devices e.g., telephones, calculators, computers, typewriters, etc., currently take on several forms. They may be constructed of plastic, silicon, rubber, or silicon rubber and can be in the form of "tree keypads" or one single unit known as "mat keypads". The graphics, e.g., numbers, letters, symbols, etc., are generally screened printed onto the keypad after it is formed. In relation to the silicone rubber "mat keypads", the ink used in the screened printing is a silicone ink. However, silicone ink only prints onto silicone and will not print onto plastic e.g., polycarbonate.
A major manufacturer of data input devices recently issued a request for companies to come up with a keypad for mobile phones that was primarily constructed of plastic rather than silicone rubber. A keypad constructed of plastic would reduce the cost of producing the keypad. The key specification of the new keypad was that it must withstand a wear resistance test in a Norman tester of 40 cycles before the ink on the keypad failed on abrasion testing.
The immediate problem with this request was that if the ink was on the top of the plastics then the ink would not withstand the Norman tester requirements as the ink would flake and wear quickly. The present inventor had intimate knowledge of silicone keypads and the Norman tester. It was, therefore, decided that to avoid the problems associated with using first surface print, i.e., the ink on the surface of the plastic, and that second surface print techniques should be used.
Second surface print is the process whereby the print is put onto the underside of the plastic sheet and not on the tops i.e., the print is viewed through the sheet rather than on top of it. The use of second surface print avoids the abrasion problems as the ink does not come into contact with anything in use.
Second surface print would be onto a flat sheet. The flat sheet would then have to be distorted or formed in order to create an acceptable keypad with raised keys. In order to align the art work correctly, manipulation and distortion techniques would be used so that the graphics would appear correctly onto the keys in question.
The second more pressing problem was that conventional ink will not stretch. Distortion of the plastic sheet would lead to pin holing whereby gaps appear in the ink. Therefore, a more flexible ink was needed.
Personnel from GE Plastics, Structured Products, have produced a document entitled "In-Mold Decoration", M. Laurin, Apr. 14, 1997, the entire contents of which are hereby incorporated by reference and relied upon. This document gives a very good background into the technology available prior to the work of the present inventor (see, for example, page entitled "Ink Selection"). Researchers at GE Plastics were asked at a seminar in America whether it was possible to produce such a flexible ink for the use required by the present inventor. The GE Plastics document lists that there are very few suitable inks for second surface decoration and there were a few companies which specialized in ink that would adhere to a flexible polycarbonate. However, no one had an ink suitable for second surface decoration to satisfy the needs for the keypads of the present invention.
After extensive experimentation, the present inventor discovered the plastics, ink, and process necessary to create a plastic keypad capable of withstanding a wear resistance test in a Norman tester of 40 cycles before the ink on the keypad failed abrasion testing requirements. The resulting keypad and method for its manufacture are the subject of the present application and more fully described below.